The present invention relates to a positive active material for secondary battery, a process for the preparation thereof and a non-aqueous secondary battery comprising such a positive active material.
As a small-sized power supply for cellular phone, video camera, etc., or large-sized power supply for electric vehicle and large-sized power supply for load-leveling of electric power, a non-aqueous secondary battery having a high energy density and a high output density attracts much attention in recent years. This non-aqueous secondary battery uses a lithium-transition metal oxide as a positive active material, and graphite, an amorphous carbon, an oxide, a lithium alloy or metallic lithium as a negative active material. However, the lithium-transition metal oxide to be used as a positive active material, e.g., lithium cobaltate (LiCoO2) is expensive. Accordingly, in order to cope with the consumption of a huge amount of non-aqueous secondary batteries which is expected to occur in the future, it is important to develop positive active materials which are inexpensive and can be prepared from abundant materials. Oxides containing manganese, nickel or iron are now under extensive studies as a positive active material for non-aqueous secondary battery. Among these materials, iron is a material which is most inexpensive and environmentally friendly. Thus, iron-based compounds are very attractive as a next-generation positive active material for non-aqueous secondary battery.
As a positive active material for non-aqueous secondary battery, there have been studied and reported various iron-based compounds. In particular, vanadium-iron composite oxides such as Fe0.12V2O5.16 (J. Power Sources, 54, 342 (1995)) and amorphous FeVO4 (DENKI KAGAKU, 61, 224 (1993)) have higher discharge capacity than other iron-based oxides such as LiFeO2 (J. Electrochem. Soc., 143, 2435 (1996)), LiFePO4 (J. Electrochem. Soc., 144, 1609 (1997)), and xcex2-FeOOH (J. Power Sources, 81-82, 221 (1999)), and thus have attracted attention as a next-generation positive active material.
However, vanadium is highly toxic as compared with iron. Thus, the content of vanadium, if used in the form of vanadium-iron composite oxide as a positive active material, is preferably controlled as low as possible. From this standpoint of view, among vanadium-iron composite oxides which have heretofore been reported, amorphous FeVO4, which has a relatively less vanadium content, is preferable as a positive active material for secondary battery.
However, M. Sugawara et al. reported that when amorphous FeVO4 is used as a positive active material for non-aqueous secondary battery, the resulting battery shows a discharge capacity drop to the level of 80% of the initial value at 5th cycle (DENKI KAGAKU, 61, 224 (1993)). In other words, amorphous FeVO4 has disadvantage that it has poor cycle life performance. Accordingly, no vanadium-iron composite oxides which have a high capacity, good cycle life performance and a low vanadium content have been heretofore found.
It is therefore an object of the present invention to provide a secondary battery which exhibits a high capacity and good cycle life performance and is inexpensive and environmentally friendly.
The positive active material for secondary battery according to the invention comprises O, Fe in an amount of higher than 25% by weight (based on the total weight of the positive active material), and V in an amount of from higher than 4% by weight to less than 35% by weight (based on the total weight of the positive active material). The content of Fe is preferably higher than 33% by weight (based on the total weight of the positive active material). Further, the content of V is preferably less than 30% by weight (based on the total weight of the positive active material). By controlling the content of Fe to be higher than 33% by weight (based on the total weight of the positive active material) and the content of V to be less than 30% by weight (based on the total weight of the positive active material), the positive active material according to the invention can be more environmentally friendly than the conventional active materials.
Further, the positive active material of the invention, when free of lithium (i.e., before charge and discharge), shows the following main diffraction peaks when subjected to X-ray analysis using CuKxcex1 rays: a peak within a 2xcex8 range of from greater than 26xc2x0 to less than 29xc2x0 (26xc2x0 less than 2xcex8 less than 29xc2x0) and a peak within a 2xcex8 range of from greater than 29xc2x0 to less than 32xc2x0 (29xc2x0 less than 2xcex8 less than 32xc2x0). The term xe2x80x9cmain diffraction peakxe2x80x9d as used herein is meant to indicate the strongest and the second strongest diffraction peaks.
The positive active material of the invention which has been subjected to charge and discharge shows a changed crystal structure and at least the following diffraction peaks when subjected to X-ray analysis using CuKxcex1 rays: a peak within a 2xcex8 range of from greater than 18xc2x0 to less than 20xc2x0 (18xc2x0 less than 2xcex8 less than 20xc2x0), a peak within a 2xcex8 range of from greater than 21xc2x0 to less than 23xc2x0 (21xc2x0 less than 2xcex8 less than 23xc2x0), a peak within a 2xcex8 range of from greater than 25xc2x0 to less than 27xc2x0 (25xc2x0 less than 2xcex8 less than 27xc2x0) and a peak within a 2xcex8 range of from greater than 31xc2x0 to less than 33xc2x0 (31xc2x0 less than 2xcex8 less than 33xc2x0). The positive active material of the invention after charged to 4.3 V vs. Li/Li+ shows these four main diffraction peaks at these four 2xcex8 ranges, respectively, when subjected to X-ray analysis using CuKxcex1 rays within a 2xcex8 range of from greater than 18xc2x0 to less than 70xc2x0 (18xc2x0 less than 2xcex8 less than 70xc2x0). The term xe2x80x9cmain diffraction peakxe2x80x9d as used herein is meant to indicate the strongest, the second, the third, and the fourth strongest peaks.
The positive active material of the invention is prepared by a process which comprises heating an aqueous solution wherein iron(III) chloride (FeCl3) and a vanadium salt are together dissolved therein at a temperature of from 40xc2x0 C. to 100xc2x0 C. for hydrolysis. This preparation process is as extremely simple as comprising a step of dissolving iron(III) chloride and a vanadium salt in water, and the step of heating the resulting aqueous solution at a temperature of from 40xc2x0 C. to 100xc2x0 C., and thus is extremely excellent as an industrial mass production process.
In this preparation process, it is preferred that the vanadium salt be VOSO4 and that an aqueous solution having FeCl3 and VOSO4 dissolved therein at a molar ratio of higher than 0.034 be used (i.e., 0.034 less than  (VOSO4/FeCl3)). The purpose of this arrangement is to prevent possible by-production of xcex2-FeOOH having much lower discharge capacity than the active material of the present invention.
The non-aqueous secondary battery comprising a positive electrode containing a positive active material of the present invention exhibits a high capacity and good cycle life performance and is inexpensive and environmentally friendly.